He also said the U.S. is concerned about the status of chemical weapons and portable anti-aircraft weapons in Syria, in light of the civil conflict there. The U.S. is open to working with a successor Syrian government to account for and destroy those weapons.

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Washington is concerned that the country's unconventional weapons could fall into the hands of terrorist or militant groups while the 11-month-old uprising continues.

Countryman drew a parallel with allied efforts to secure loose weapons in Libya during the fighting there, but called the situation in Syria "much more difficult."

Meanwhile in Syria, the military unleashed a new offensive in Hama, a city with a bloody history of resistance to Assad's late father Hafez al-Assad, firing at residential neighborhoods with anti-aircraft guns mounted on armored vehicles, opposition activists said.

Artillery also shelled parts of Homs for the 13th day in a row. In Damascus, troops backed by armor swept into the Barzeh district, searching houses and making arrests, witnesses said.

International efforts to halt the carnage have faltered. France said it was negotiating a new U.N. Security Council resolution on Syria with its ally Russia, and also wanted to create humanitarian corridors to ease the plight of civilians caught up in the violence.

French Foreign Minister Alain Juppe said a U.N. General Assembly vote on Thursday on a non-binding resolution on Syria would be "symbolic". It follows a Feb. 4 veto by Russia and China of a draft Security Council resolution that backed an Arab League call for Assad to quit.

Russian Foreign Minister Sergei Lavrov said he would listen to Juppe's views, but added: "If the plan is to use the Security Council and United Nations to adopt some language to help legitimize regime change, then I'm afraid international law does not allow this and we cannot support such an approach."

Lavrov said later in the day: "If leading members of the international community demand regime change as a condition for everything else, then we are convinced ... this is the way to a full civil war with unforeseeable consequences."

The Arab League also wants a joint U.N.-Arab peacekeeping force to be deployed in Syria and has adopted a resolution that would allow its members to arm Syrian rebels.

Western powers are keen to see Assad go but are wary of intervening in a country at the heart of a volatile region.